Military alliances are related to collective security systems but can differ in nature. An early 1950s memorandum from the United States Department of State explained the difference by saying, that historically alliances "were designed to advance the respective nationalistic interests of the parties, and provided for joint military action if one of the parties in pursuit of such objectives became involved in war". While a collective security arrangement "is directed against no one; it is directed solely against aggression. It seeks not to influence any shifting 'balance of power' but to strengthen the 'balance of principle'".
The obvious motivation in states engaging in military alliances is to protect themselves against threats from other countries. However states have also entered into alliances to improves ties with a particular nation or to manage conflict with a particular nation.
The nature of alliances, including their formation and cohesiveness (or lack thereof), is a subject of much academic study past and present, with the leading scholars generally considered to be Glenn H. Snyder and Stephen Walt.
In the European historical context, a military alliance can be viewed as a league between independent states, defined by treaty, for the purpose of combined action, defensive or offensive, or both. Alliances have often been directed to specific objects carefully defined in the treaties. Thus the Triple Alliance of 1668 between Great Britain, Sweden and the Netherlands, and the Grand Alliance of 1689 between the Holy Roman Empire, Holland, England, Spain and Saxony, were both directed against the power of Louis XIV of France. The Quadruple or Grand Alliance of 1814, defined in the Treaty of Chaumont, between Great Britain, Austria, Russia and Prussia, had for its object the overthrow of Napoleon and his dynasty, and the confining of France within her traditional boundaries. The Triple Alliance of 1882 between Germany, Austria and Italy was ostensibly directed to the preservation of European peace against any possible aggressive action of France or Russia; and this led in turn, some ten years later, to the Dual Alliance between Russia and France, for mutual support in case of any hostile action of the other powers.